The present invention relates to high purity copper and a method of producing high purity copper based on electrolysis. The high purity copper produced with the method of the present invention can be used to produce high purity copper alloy by adding the necessary alloy elements. The present invention covers all of the above. Incidentally, “%” and “ppm” as used in this specification respectively represent mass % and mass ppm. Moreover, “purity” represents the purity excluding C, O, N, and H as gas components.
Conventionally, if the aim is to produce high purity copper, emphasis was primarily placed on eliminating metal elements (excluding copper) and nonmetal elements, which are recognized as impurities, and limiting gas components to a constant amount of several ppm to several 100 ppm. Thus, trace amounts of inclusions that existed in the high purity copper were not acknowledged as a problem, and no consideration was given for eliminating or reducing the same. Moreover, even in cases where gas components are limited as much as possible, there was no concern with respect to the mode of existence of the inclusions arising therefrom.
Nevertheless, if there are inclusions other than copper in the high purity copper, even minute and trace amounts, for example, may cause disconnection with such inclusions as the source in the thinning process of the copper bonding wire or problems may arise regarding mechanical properties such as tensile properties, and adverse effects may also be inflicted on the reproducibility of such properties. In addition, when preparing a high purity copper sputtering target for use in semiconductor devices, protrusions (nodules) would arise on the target surface in the process of forming a thin film by way of sputtering, and particles would arise due to the rupture or the like of the protrusions (nodules) caused by an abnormal discharge. Generation of particles causes the percent defective of the semiconductor device to deteriorate.
Conventionally, the recognition was that other causes had a greater influence on the generation of the foregoing particles or the rupture of the bonding wire, and the recognition that minute and trace amounts of inclusions existing in the high purity copper target were the cause was low. Nevertheless, as the conventionally recognized sources of particles and causes of rupture of the bonding wire become clarified and subsequently solved, the new recognition is that other sources of particles exist and that, unless they are solved, it is not possible to realize high quality deposition or obtain a bonding wire that will not rupture easily. To put it differently, existing sputtering targets and bonding wires for forming copper wiring for use in semiconductors are based on the foregoing sophisticated technology level. It should be easy to understand that the high purity copper of the present invention can be applied to all materials that use high purity copper in addition to the use in sputtering targets or bonding wires.
Although the deposition technology of copper wirings or bonding wires for use in semiconductors is well known technology, the principle of the sputtering method, which is sometimes slightly difficult to understand, is briefly explained below. The sputtering method forms a film on a substrate by utilizing the phenomenon where atoms configuring the target are discharged into space and accumulated on the opposing substrate based on the momentum exchange that occurs when the accelerated charged particles collide with the target surface. The sputtering target is usually in the shape of a discoid or rectangular plate, and is used as the sputter source for forming, by way of sputtering, an electrode, gate, element, insulating film, protective film and the like of various semiconductor devices. Generally, as the sputtering target, an aluminum and aluminum alloy target, a copper and copper alloy target, a high melting point metal and alloy target, a metal silicide target and the like are used.
Among the foregoing targets, an important target is a copper and copper alloy target that is used in forming a copper wiring as an alternative of a conventional aluminum wiring. Meanwhile, during the deposition based on sputtering, protrusions having a size of several μm to several mm referred to as nodules sometimes arise on the eroded portion of the sputtering target. There is a problem in that such nodules will burst as a result of colliding with the charged particles in the sputtering process, and thereby cause to generate particles (cluster-shaped coarse fragments) on the substrate.
The generation of particles will increase in proportion to the number of nodules on the eroded surface of the target, and a major issue is to prevent the generation of nodules in order to reduce the problematic particles. In the recent circumstances where LSI semiconductor devices are subject to higher integration and the linewidth thereof being miniaturized to 0.25 μm or less, the generation of particles caused by the foregoing nodules is now being considered a major problem.
Specifically, particles directly adhere to the thin film that is formed on the substrate, or once adhere to and accumulate on the circumferential wall or component of the sputtering device and thereafter flake off and adhere to the thin film again, and cause problems such as the disconnection or short circuit of the wiring. The generation of particles is becoming a major problem pursuant to the advancement of higher integration and miniaturization of the electronic device circuit as described. As described above, causes of the conventionally recognized sources of particles and rupture of the bonding wire are being clarified and many of them have been solved, but the current situation is that it is still insufficient. With the foregoing problems unsolved, it is not possible to achieve high quality deposition or obtain a bonding wire that will not rupture easily.
Conventional technologies are now introduced. Nevertheless, the following conventional technologies have no concern regarding the mode and influence of the minute and trace amounts of inclusions existing in the high purity copper, and do not provide any kind of specific solution therefor. Japanese Published Unexamined Patent Application No. H11-106842 describes cleaning electrolyte based on solvent extraction. Japanese Published Unexamined Patent Application No. 2000-107596 describes eliminating Sb and Bi with chelate resin. Japanese Published Unexamined Patent Application No. S63-297583 describes adding a diaphragm and glue in copper electrolysis to smooth the electrolyzed surface, thereby reducing the uptake of impurities. Japanese Published Unexamined Patent Application No. S64-55394 describes causing anolyte to come in contact with activated carbon in copper electrolysis to eliminate glue. Japanese Published Unexamined Patent Application No. H1-152291 describes performing electrolysis once again in copper electrolysis. Japanese Published Unexamined Patent Application No. S64-8289 describes smoothing the electrode surface based on periodic reverse current electrolysis in copper electrolysis to prevent the inclusion of suspended matter and electrolyte. Japanese Published Unexamined Patent Application No. 2005-307343 describes adding a macromolecular additive to improve the surface condition in copper electrolysis and using electrolyte containing urea to produce high purity copper with a low silver and sulfur content.
Japanese Published Examined Patent Application No. 2004-513228 describes that the three metallurgical characteristics of a sputtering target that affect the performance of a target are the uniformity of the material (no precipitate, void, inclusion and other defects), crystal particle size (finer crystal particle size is generally more preferable than coarse crystal particle size), and texture (texture relates to the strength of a specific crystallographic orientation; a “weak” texture includes substantially random distribution of the crystallographic orientation, and a “strong” texture includes a preferential crystallographic orientation in the distribution of the crystallographic orientation), and further states that, generally speaking, it is necessary to reduce defects such as inclusions in the target.
Japanese Published Unexamined Patent Application No. H5-214519 describes a titanium sputtering target in which the number of inclusions of 1 μm or more existing at the crystal grain boundary of titanium configuring the target is 100 inclusions or less per 1 cm2 of the target plane, and additionally describes that the inclusions existing at the crystal grain boundary of titanium are a composite compound based on a combination of one or more types among oxides, nitrides, carbides, sulfides, and hydrides of metal components of titanium or iron, nickel, chromium, aluminum, silicon, tungsten, and molybdenum, and that the oxides can be decomposed by heat treatment.
Japanese Published Unexamined Patent Application Nos. H9-25564 and H11-315373 describe reducing the number of inclusions in an aluminum or aluminum alloy target to be 40 inclusions/cm2 or less per unit area, that splashes can be reduced by causing the maximum length of the inclusions to be 20 μm or less, that reducing the inclusions in the sputtering target is particularly important in order to inhibit the generation of particles and splashes, and reducing inclusions by filtering molten metal with a ceramic filter.
Japanese Published Unexamined Patent Application No. 2000-239836 describes a method of producing a high purity copper or copper alloy sputtering target having an oxygen content of 100 ppm or less, carbon content of 150 ppm or less, nitrogen content of 50 ppm or less, and sulfur content of 200 ppm or less, wherein used is a high purity copper or copper alloy ingot in which the oxygen content in the target is 100 ppm or less, the carbon content is 150 ppm or less, the nitrogen content is 50 ppm or less, and the sulfur content is 200 ppm or less, or the number of indications having a flat bottom hole diameter of 0.5 mm or more is 0.014 indications/cm2 or less in an ultrasonic inspection performed from the target surface, and obtained by melting and casting based on electron beam melting or vacuum induction skull melting. However, the large inclusions detected in the ultrasonic inspection are not observed in current high purity copper targets.
WO 2004/083482 describes that the gas components of oxygen, nitrogen, and carbon contained in the copper alloy sputtering target form inclusions at the crystal grain boundary and cause the generation of particles, and that it is desirable to reduce such gas components as much as possible since they cause the unexpected generation of particles during the sputter life, and unavoidable impurities excluding gas components are reduced to 10 wtppm or less.